Antennas have been long used as devices for converting a high-frequency current into an electromagnetic wave and an electromagnetic wave into a high-frequency current. The antennas are categorized into subgroups such as linear antennas, planar antennas, and solid antennas, based on their shapes. The linear antennas are further categorized into subgroups such as a dipole antenna, a monopole antenna, and a loop antenna. A dipole antenna having a linear antenna element has a significantly simple structure (see Non-patent Literature 1), and is now used widely as a base station antenna etc. Further, there has been known a planar dipole antenna which includes a planar antenna element in place of the linear antenna element (see Non-patent Literature 2).
(a) of FIG. 30 illustrates a structure of a conventional dipole antenna dp. The dipole antenna dp includes (i) a linear antenna element e1 extending from a feed point F in a first direction, and (ii) a linear antenna element e2 extending from the feed point F in a direction which is opposite to the first direction. The dipole antenna dp serves as a transmitting antenna for converting a high-frequency current into an electromagnetic wave or a receiving antenna for converting an electromagnetic wave into a high-frequency current. Note, however, that a high-frequency current (electromagnetic wave) that can be efficiently converted into an electromagnetic wave (high-frequency current) by use of the dipole antenna dp is limited to the one which has a frequency in the vicinity of a resonance frequency of the dipole antenna dp.
(b) of FIG. 30 illustrates current distribution (fundamental mode) at a first resonance frequency f1 of the dipole antenna dp. At the first resonance frequency f1, a direction in which a current flows through the antenna element e1 and a direction in which a current flows through the antenna element e2 are identical with each other (see (b) of FIG. 30). Accordingly, in a case where a high-frequency current having a frequency in the vicinity of the first resonance frequency f1 is received via the feed point F, an electromagnetic wave having a single-peaked radiation pattern is radiated from the antenna elements e1 and e2.
(c) of FIG. 30 illustrates current distribution (higher order mode) at a second resonance frequency f2 of the dipole antenna dp. At the second resonance frequency f2, a direction in which a current flows through the antenna element e1 and a direction in which a current flows through the antenna element e2 are different from each other (see (c) of FIG. 30). More specifically, two points in antenna elements e1 and e2, indicating a ⅓ point of an entire length of a combined antenna elements e1 and e2 and a ⅔ point of the entire length, respectively, serve as two nodes of the current distribution, so that a direction in which current flows through the antenna elements e1 and e2 is inverted at each of the two nodes. For this reason, in a case where a high-frequency current having a frequency in the vicinity of the second resonance frequency f2 is received via the feed point F, an electromagnetic wave having a split radiation pattern is radiated from the antenna elements e1 and e2. This is because electromagnetic waves radiated from sections of the antenna element f1 and sections of the antenna element f2 interfere with each other so that an intensity of an electromagnetic wave is significantly weakened in a specific direction as compared with the other directions.